This invention relates to a method and apparatus for positioning a pipe on the sea floor, for laydown or abandonment, and recovering a pipe laid on the sea floor.
It is well known to lay pipe on the sea bed for conveying oil, gas and the like between two sites, such as a production well and adjacent production platform. In order to lay the pipe, a pipelaying vessel is used which typically discharges the pipe from a pipe storage drum on the vessel down into the sea while the vessel is advancing on the sea surface. Typically, the pipe adopts the shape of a catenary between the pipelaying vessel and the sea bed. The pipe may be a flexible pipe; alternatively, it may be a rigid (metal-walled) pipe, which requires to be passed through a straightener on the vessel, after it has been drawn off the pipe storage drum, in order to remove the plastic deformation that it has to undergo when it is wound onto the drum initially.
During pipelaying operations, it is necessary to complete the laying of each pipe length by lowering onto the sea floor the surface end (i.e. the second end) of the pipe that is being laid. This operation is known as either abandonment or lay down, depending on the circumstances.
In the case of abandonment, the pipe end is lowered to the sea bed and left there with the intent of subsequently recovering it to continue the previously interrupted pipelaying operations, for example due to adverse weather conditions. It should be noted that if abandonment becomes necessary when only part of the pipe length has been laid, it is necessary to cut the pipe and abandon the cut end.
In the case of lay down, recovery of the pipe end from the sea bed is not intended, because the pipe is to become part of a permanent installation—e.g. it is to be buried, or connected to other subsea equipment. In this case, the surface end of the pipe may or may not be fitted with some form of end attachment—e.g. collet connector, Pipeline End Termination (PLET), pig launcher etc.—prior to being lowered to the sea bed.
It will be appreciated that lowering of the pipe end to the sea bed, can be a planned operation such as would be effected at the end of a pipelaying operation (lay down), or when the vessel has to return to a shore side spooling base for a further load of pipe (abandonment). Alternatively, it may have to be undertaken in certain unplanned circumstances, such as during bad weather when the vessel has to leave the work site (abandonment).
Following abandonment, the vessel will return at a suitable later time to resume pipelaying. The pipe end has to be recovered from the sea bed, before pipelaying can recommence.
To perform the operations of lay-down, abandonment and recovery, an Abandonment and Recovery (A&R) winch has conventionally been used. The winch line, which may take the form of wire, cable, or synthetic rope, is attached to an end portion of the pipe and the winch is operated to lower and lift the pipe, as required. The A&R winch has to be rated, so as to be able to carry the loading of the longest length of a given pipe likely to extend from the pipelaying vessel to the sea floor. If the water is very deep, then the pipe length reaching from the sea bed up to the vessel will exert a correspondingly high loading on the A&R winch. For example, if the sea water has a depth of 3000 meters, the suspended pipe length could weigh about 400 tons (406 tonnes). Accordingly, the A&R winch has to be rated to at least this capacity. It also needs to have storage capacity for at least 3000 meters (3 kilometers) of the large, heavy and very expensive wire that has to be used for lowering the pipe to the sea floor.
In practice, since A&R operations (as well as lay-down operations) generally take place at depths considerably shallower than 3000 meters and since the necessary operations cannot be handled as conveniently with a 400 ton (406 tonne) winch as with a 200 ton (203 tonnes) winch, the lay vessel is typically equipped with a smaller capacity A&R winch as well. For example, a vessel equipped with a 400 ton (406 tonne) A&R winch might typically also be provided with a 200 ton (203 tonnes) A&R winch, for abandonment and recovery operations at relatively less deep sea floor locations. The reduced wire diameter for the smaller capacity winch (for example 76 mm /3 inches) for the wire wound on a 200 ton (203 tonnes) winch facilitates the handling operations as compared with when using a larger capacity winch (for example for a 400 ton (406 tonnes) winch, which would have a wire size of about 114 mm (4.5 inches).
The disadvantage with the pipelaying equipment described is that the wire of the higher capacity winch has to be extremely strong in order that it can take the loads imposed by the pipe. This adds to cost. Furthermore, due to the weight of the large length of pipe needed to extend to depths of as much as 3000 meters and the large wire diameter needed to carry such large pipe loads, the higher capacity A&R winch has to have a large wire storage drum, which adds to the bulk and cost of the higher capacity winch.